1. Field of the Invention
This invention relates to gas turbine engines and more particularly to Helmholtz resonator-type acoustic liners for fluid-flow ducts, such as the fan duct or afterburner exhaust duct, of gas turbine engines.
2. DESCRIPTION OF THE KNOWN ART
Modern gas turbine engines operate over a broad range of speeds and thrusts thereby generating a broad range of noise frequencies. This together with ever more stringent noise abatement requirements around the world pose great challenges to the aircraft engine designer. Modern day gas turbine engine designers are always striving to design acoustic liners which are effective in absorbing noise over a broad range of frequencies and yet economical with respect to the space they occupy. Further design considerations are durability, weight and ease of fabrication. Particular applications for these acoustic liners are in the fan ducts and exhaust ducts of gas turbine engines. Another application is in the exhaust duct of a gas turbine engine incorporating an augmentor, wherein the acoustic liner is used to overcome a particularly difficult noise problem referred to as screech.
A most commonly used type of acoustic liner is multilayer-type, in which a bulk fiber-filled or compartmented-airspace core is provided with a permeable core facing laYer. The core facing layer is usually a sheet fabricated from sintered or felted metal or other porous materials having relatively large flow resistances. The drawback to this type of acoustic liner is its poor durability serviceability and the large thickness required to suppress low frequency noise as encountered in afterburner screech.
This problem can be overcome with the use of Helmholtz resonator chamber or compartments as in U.S. Pat. No. 3,819,007 which provides a compartmented airspace core with a permeable facing layer having apertures through it, each aperture having a collar in the form of a short duct, one end of which is flush with the outer (sound receiving) surface of the facing sheet, the other end of which projects into the interior of a respective compartment of the core. This type of Helmholtz resonator with the neck situated internally of the resonant volumes and perpendicular to the flow provide good noise attenuation over a relatively narrow band of low frequencies centered about a particular low frequency to which the resonators are tuned. Economy with respect to space is improved due to the fact that the Helmholtz resonator necks are internal of the resonant volumes.
However, a problem still remains if it is desired to efficiently absorb a broader band of low frequencies using such an acoustic liner, due to the relatively narrow-band frequency response of the Helmholtz resonators. Furthermore, the design requires resonator chamber depths and neck lengths that add significant radial width to the ducts thereby incurring additional unwanted weight and size. In particular the available designs of the prior art add significant thickness to afterburner liners in the exhaust ducts of high performance gas turbine engines thereby adding to the diameter installation envelope.